Many of the diseases of the immune system, particularly the rheumatic diseases, predominate in women. Various reasons for these differences are proposed. Included in this paper are the differences between men and women's immune system and the sex hormone and gonadotrophin effects on immunity. The hydroxylation of estrogen is discussed in the context of lupus erythematosus, and various hormones such as progesterone and the androgens are compared with regard to their effects in the normal host and the woman with disease.